This invention relates generally to construction equipment and, in particular, to methods and apparatus for lifting open-web steel trusses.
It is well known in the commercial construction industry to use standard prefabricated steel Warren trusses to support floors and roofs of structures such as office buildings and hotels where loads are moderate and the spans between supports are relatively long. These trusses, often referred to as open-web steel joists are generally made of light structural members such as angles, bars and channels.
Open-web steel joists are commonly manufactured in three standard categories. The standard K-series joists are fabricated using a double-angle top and bottom chord and a round bar web having a depth of from 8 inches to 30 inches. K-Series joists are recommended for spans from 8 feet to 60 feet in length. Other standard open-web steel joists include the LH-series which have a depth of from 18 to 48 inches and may be used for spans of 25 feet to 96 feet and the DLH-series joists which have a depth of from 52 inches to 72 inches and are recommended for spans of 89 feet 244 feet.
Open-web steel joists are very economical structural members since they are fabricated from standard lightweight structural steel shapes such as angles and bars. Because the webs are open, they are able to span long distances without the dead weigh load of a solid I-beam. Moreover, because of the open-web design, is possible to run plumbing, electrical lines and ventilation ducts directly through the web itself, which results in considerable savings in floor-to-floor height and weight.
Although open-web steel joists have very favorable strength to weight ratio for vertical loads (i.e. loads applied parallel to the depth, which is the axis having the maximum area moment of inertia), open web steel joists have considerably less strength when resisting side loads (i.e. loads applied to the axis having the minimum area moment of inertia). Consequently, open-web steel joists must be handled carefully especially during loading, transport, unloading and positioning prior to final placement.
The Steel Joist Institute recommends that when lifting an open-web steel joist using a crane (either during loading, unloading or during final placement), the crane operator should use two chokers configured in a basket hitch with two-way spreaders. The chokers should be rigged passing through the inside of the inverted V-shaped opening in the web. The Institute cautions that when using chokers (with or without spreaders), care must be taken to avoid damaging (e.g. bending) the rod members that form the web. Carefully rigging and unrigging chokers and spreaders, however, is cumbersome and time-consuming. Accordingly, a need exists for a method and apparatus to quickly rig and unrig the open-web steel joists from the lifting crane.
One prior art apparatus, marketed commercially as the E-Z Joist Release™ by Freedom Tools LLC of Mesa Ariz., comprises a horizontal flange welded to a vertical web adapted to receive a lifting hook or shackle. The horizontal flange has a hole at each end through which a vertical rotating shaft is mounted. Each of the rotating shafts has a shank that is sized to pass through the gap between the structural angles that make up the top flange of the truss. The rotating shafts terminate at their lower ends with an inverted triangular tip, which when rotated 90° is unable to pass through the gap in the top flange. The horizontal flange also has two vertical tongues adjacent the rotating shafts. The vertical tongues are also sized to pass through the gap in the top flange and serve as guides to key the device to the top flange of the truss. In operation, the device is placed on top of the truss so that the tongues and rotating shafts pass through the gap in the top flange. A lever attached to the rotating shafts is pulled which causes the shafts to rotate 90° to lock the device in position. Once the truss has been moved to its desired location, the lever is returned to its original position so the device can be released from the truss. The E-Z Joist Release has several disadvantages. It is expensive. It may not be easily adaptable to joists having different-size upper flanges because the rotating tip is at a fixed depth. Additionally, the E-Z Joist Release may damage the web members if the tongues and/or rotating shafts come in contact with the web.
Accordingly, what is needed is a joist lifting tool that is inexpensive, easy to use, and safe.